The present invention is related to fluid pressure operated brake actuators and particularly to transit type railway car brake actuators having a spring-applied, fluid pressure released auxiliary brake for supplementing or backing up the service brake.
These spring-applied brakes are fail-safe in the sense that spring force is immediately available to effect a brake application in the event of a malfunction causing loss of pneumatic or hydraulic pressure, which normally acts on a piston to counteract the spring load. In addition, the spring brake provides a convenient and positive parking brake. It will be appreciated, however, that in the absence of any fluid pressure to retract the spring following actuation thereof, the vehicle brakes are locked up, thus requiring a separate means of releasing the spring-applied brake before the vehicle can be moved.
Various manual release means have been proposed to retract the spring and release the brake without requiring pneumatic or hydraulic fluid pressure. These arrangements have not proven entirely satisfactory, however, since it is incumbent upon the operator to release the manual retraction means following reapplication of pneumatic or hydraulic pressure. Should the operator fail to carry out this requirement, the spring will be held in a compressed state by retraction means, thereby preventing the spring-applied brake from being subsequently reapplied, in the event of pneumatic or hydraulic pressure loss.